Improving the dielectric temperature stability of Bi₂SiO₅-based ceramics through the spontaneous formation of paraelectric–ferroelectric nanocomposite structures

The substitution of a portion (∼3%) of Bi³⁺ in ferroelectric Bi₂SiO₅ ceramics with La³⁺ induces a paraelectric phase and stabilizes the dielectric permittivity by eliminating the ferroelectric-paraelectric phase transition. However, the non-negligible negative temperature dependence of the permittivity remains an issue for practical applications in capacitors, resonators, and antennas. Herein, we show that the additional substitution of Si⁴⁺ with Ge⁴⁺ in paraelectric (Bi_0.97La_0.03)₂SiO₅ can substantially improve its dielectric temperature stability. Ceramic samples with compositions of (Bi_0.97La_0.03)₂Si_1-<i>x</i>Ge_<i>x</i>O₅ with <i>x</i> up to 0.3 were prepared <i>via</i> a sol-gel process and subsequent low-temperature sintering below 720 °C. The incorporation of Ge stabilized the ferroelectric phase with an elevated Curie temperature, leading to the spontaneous formation of a paraelectric-ferroelectric nanocomposite structure. The fraction of the ferroelectric phase increased from 0% at <i>x</i> = 0 to 53% at <i>x</i> = 0.3. Because of the negative and positive temperature dependence of the paraelectric and ferroelectric phases, respectively, the sample with <i>x</i> = 0.2 exhibited a dielectric permittivity over 50 with a small temperature coefficient of -70 ± 50 ppm °C^-1 in a temperature range from -55 to 125 °C. The ceramics also showed a paraelectric-like linear polarization response under electric fields up to 280 kV cm^-1.